Tool holders are used in various machine tools, such as turning centers and lathes, for the purpose of holding a variety of different tools such as boring bars. Boring bars are typically used by a turning center machine tool during a boring operation, the purpose of which is to increase the size of pre-existing internal features of a workpiece while obtaining target size and surface finish accuracy. Under very rigid tool setups, boring operations are often capable of maintaining a size tolerance within 0.0002″.
A turning center machine tool includes a workpiece spindle for holding and spinning a workpiece, and may further include a tool spindle, tail-stock, or tool turret for holding a tool holder and tool. A tailstock typically includes a socket into which a rearward end of the tool holder is inserted. Some tool holders have a generally cylindrical shank that extends forward from the rearward end and that includes one or more longitudinal flats for orienting the tool holder within the tailstock socket and for use with a setscrew fastening arrangement. The shank typically terminates at a mounting flange for axially locating the tool holder against an outboard face of the machine tool tailstock. Forward of and adjacent the mounting flange there is provided a set screw diameter through which set screws radially extend and intersect with a tool bore that axially extends through the tool holder for accepting a boring bar therein.
The boring bar is typically a generally cylindrical tool having a fastening end that inserts into the tool bore of the tool holder. Extending forward from the fastening end, the boring bar includes a solid shank having one or more longitudinal flats against which the tool holder set screws are fastened for holding the boring bar within the tool bore of the tool holder. Extending further forward, and opposite the fastening end, the boring bar terminates in a seat portion into which a cutting insert fastens.
The distance between the tip of the cutting insert and an outboard face of the set screw diameter of the tool holder defines what is known as the unsupported overhang of the boring bar. In general, the greater the ratio between the length of the unsupported overhang to the diameter of the boring bar—the lesser the rigidity of the tool setup. Lesser tool rigidity results in tool vibration and chatter, thereby necessitating reductions in machining federates and throughput in order to maintain workpiece accuracy and surface finish.
Boring of workpieces having stepped diameters, or variably sized internal features, presents a special problem for boring tools. A stepped diameter workpiece is one having a shallower, larger diameter and one or more deeper, smaller diameters. The shallower, larger diameter of the workpiece is relatively proximate the tool holder, thereby requiring very little unsupported overhang of the tool and permitting a more rigid and larger diameter boring bar to be used. Thus, the shallower, larger diameter can often be cut relatively quickly and accurately due to the rigidity of the tool setup. In contrast, the deeper, smaller diameters are relatively distal the tool holder, thereby necessitating longer unsupported overhang and smaller diameter of the boring bar.
To reach the deeper, smaller diameters, the machining process must be interrupted to change from a rigid, larger diameter boring bar to a smaller diameter boring bar having a longer unsupported overhang. Such an interruption is a major risk to workpiece accuracy for at least a couple of reasons. First, using a smaller diameter boring bar sacrifices tool rigidity due to a corresponding decrease in cross-sectional surface area and beam strength of the tool. Second, a tool change disrupts the dimensional relationship between the shallower, large diameter and the deeper, smaller diameter since two different tools must be used. Simply put, using two different tools is undesirable since the subsequent tool will not necessarily pick up exactly where the original tool left off in the cut. Conversely, it is desirable to use the same single tool to cut both diameters to maintain continuity of the cut and thereby more strictly maintain the dimensional relationship between the diameters.
Moreover, using two different tools results in increased manufacturing time and costs. First, interrupting the machining operation to execute a tool change results in increased machine cycle time. Second, the deeper, smaller diameters must be cut magnitudes more slowly than the shallower, larger diameters. Slow machining is necessary to maintain the same size accuracy throughout the workpiece when cutting the workpiece with the smaller diameter boring bars having long unsupported overhangs. In a manufacturing environment, every second of cycle time is accounted for. Thus, where time is money, unnecessarily slow machining performance translates into unnecessarily high manufacturing costs.
Another special problem with boring involves the interconnection of carbide boring bars to boring bar tool holders. Carbide boring bars are cylindrical and have longitudinal flats extending therealong to facilitate setscrew fastening to a respective tool holder. The longitudinal flats present two problems. First, incorporating longitudinal flats along a boring bar yields less tool rigidity. The longitudinal flats require a loss in cross-sectional area and beam strength. Second, setscrews do not always squarely engage the longitudinal flats of the boring bar despite being tightly fastened down. In other words, the boring bar can be clocked such that the flat become out of square with the set screws wherein the boring bar can work loose from engagement with the set screws during machine operations. Third, setscrews only engage a small area of the boring bar roughly equal to the diameter of the point of the setscrew. The above problems manifest themselves in the form of unnecessarily compromised rigidity of the boring bar tool setup, and an attendant decrease in workpiece quality and/or increase in machining time and cost.